At present, optical elements are used in various fields. Depending on the purpose for which they are used, it is difficult to materialize required optical characteristics and so forth in some cases in respect of conventional spherical lenses comprised only of glass. Accordingly, resin-cemented optical elements comprising a base member provided thereon with a cured resin layer having a stated shape are attracting notice.
For example, in order to make optical elements such as camera lenses compact and light-weight, it is important to lessen the number of component lenses of an optical system. In order to lessen the number of component lenses, it is effective that a component part constituted of a plurality of spheric lenses is replaced with one aspheric lens.
“Aspheric lens” is a generic term for lenses the curvature of which is kept continuously different over the region extending from the lens center toward the periphery. The use of aspheric lenses at some part of optical systems enables considerable reduction of the number of lenses necessary for the correction of aberrations, compared with a case in which the optical system is constituted only of spheric lenses. This can make the optical system compact and light-weight. Also, the use of aspheric lenses enables high-grade correction of aberrations which is difficult for spherical lenses, and hence can bring about an improvement in image quality.
Aspheric lenses having such superior characteristics have not necessarily come into wide use. The greatest reason therefor can be said to be a difficulty in working. Conventional aspheric lenses make use of base members made of glass, and have only be able to be produced by precisely polishing this glass, having involved the problem of a high cost.
In recent years, however, techniques for producing resin-cemented optical elements such as composite-type aspherical-surface molding, plastic molding, and glass molding have been put into practical use one after another, and it has become possible to produce aspheric lenses at a low cost by these methods. Thus, the aspheric lenses have rapidly come into wide use. Nowadays, such aspheric lenses have come into wide use in camera lenses and so forth.
The plastic molding is a method in which a resin is injected into a mold with the desired aspherical shape to effect molding. This method can enjoy a high productivity and a low cost. It, however, has had problems that aspheric lenses thus produced have a limit to their refractive index and moreover are inferior to glass lenses in respect of figure tolerance and reliability.
The glass molding is a method in which a glass blank material standing softened is shaped in a mold having the desired aspherical shape. This method enables achievement of mass productivity and high precision. It, however, has a limit to the types of glass usable therefor. Moreover, it requires a relatively high molding temperature, and may impose a great load on the mold. Accordingly, how this load be reduced comes into question.
The composite-type aspherical-surface molding is a method in which, using a mold having an aspherical shape, a resin layer having the aspherical shape is provided on a spheric or aspheric glass lens. This method can be said to be a method having both the characteristics, i.e., the reliability the glass lens has and the mass productivity the plastic molding has. In the present specification, a lens produced by this composite-type aspherical-surface molding is called a PAG lens. Conventional PAG lenses have characteristic features that they can well be mass-produced and are relatively inexpensive. They, however, have problems such that, compared with aspheric lenses made of glass, they have a restriction on the extent of designable aspheric surface, have a low light transmittance, and may change in optical performance depending on environment, resulting in a poor reliability.
As the resin used in this composite-type aspherical-surface molding, it may include thermoplastic resins and photosensitive resins. In the case when aspheric lenses are produced, a method is especially effective in which a composition of photosensitive resin (photo-reactive resin) is made to adhere to the surface of a base member, followed by irradiation with light such as ultraviolet light to effect curing. However, when such a photosensitive resin is used in the PAG lens resin layer, there is a problem that the shape of the mold can not exactly be transferred especially in the case of a PAG lens having a large extent of aspherical surface, i.e., having a large resin thickness. This imposes a restriction on designing.
Conventional PAG lenses also have a lower light transmittance than glass lenses, and hence the employment of such PAG lenses may possibly lead to a low transmittance of the whole optical system. For this reason, the number of PAG lenses usable in one optical system is usually limited to one or two.
Resin-cemented optical elements such as the PAG lenses also have a problem that they may so greatly change in optical performance depending on environment as to have a poor weatherability. In order to improve the weatherability, it is effective to enhance the degree of cure (degree of polymerization) of the resin, and, in order to do so, it is effective to irradiate the resin by a large amount of light so as to cure the resin further. However, an increase in irradiation level results in a decrease in light transmittance of the resin because of its yellowing. Thus, it is difficult for any conventional techniques to achieve both the improvement in light transmittance and the improvement in weatherability of the resin-cemented optical element.